While some forms of hormone replacement therapy have been found to increase breast cancer risk, therapies that use only estrogen may actually protect women against the disease, a new study says.

Women in the study who took estrogen-only hormone replacement therapy (HRT) for six years were 23 percent less likely to develop breast cancer five years after they had stopped the therapy compared with women who never received HRT. Women who took estrogen-only HRT were also less likely to die of breast cancer.

The findings mostly apply to women who have had a hysterectomy, as this is the group most likely to take estrogen-only HRT. The therapy raises the risk of uterine cancer, so the therapy is usually not prescribed for other women.

"These latest results should provide reassurance about breast safety," for women taking estrogen for about five years after a hysterectomy for relief from postmenopausal symptoms, said study researcher Garnet Anderson, of the Fred Hutchinson Cancer Research Center in Seattle.

However, the researchers said they do not recommend estrogen-only HRT as a way to reduce breast cancer risk in general. The lowered risk was not found in women with a family history of the disease, and the therapy comes with a risk of stroke and blood clots, the researchers said.

The study will be published online March 7 in the journal Lancet Oncology.

Does HRT cause harm?

In the 1990s, the Women’s Health Initiative, a study involving more than 100,000 women, was established to determine the risks and benefits of HRT. One branch of the study looked the effects of taking estrogen and progestin together, but it was halted ahead of schedule in 2002 because the hormone combination was found to increase the risk of breast cancer by 25 percent.

A separate study branch looked at the effect of estrogen-only HRT in 11,000 postmenopausal women who had had hysterectomies. This trial was stopped in 2004 because the therapy was found to increase the risk of stroke and blood clots.

In the new study, the researchers followed 7,645 women from the estrogen-only branch for about five years after the trial ended. About half had taken estrogen during the trial, and half had taken the placebo.

Over the course of the 11-year study, 151 women who took estrogen-only HRT developed breast cancer compared with 199 women took a placebo. Six women who took estrogen-only HRT died from breast cancer, compared with 16 who took a placebo.

Estrogen confusion

"It's very, very reassuring," Dr. John Buster, professor of obstetrics and gynecology at Brown University, who was not involved in the study, said of the findings. "It really takes the fire out of the argument that estrogens cause breast cancer," Buster said.

Some previous studies have suggested that estrogen-only HRT increases the risk of breast cancer, but these studies were less rigorous in design than the new one.

In addition, women taking HRT in the earlier studies may have also undergone more frequent mammograms, making it more likely breast cancer would be detected. In the new study, women who received HRT were just as likely to get a yearly mammogram as those who did not receive HRT, the researchers said.

It's still not clear why estrogen-only therapies and therapies that use a combination of estrogen and progestin appear to have the opposite effect on breast cancer risk.

The study was funded by the National Heart, Lung and Blood Institute and Wyeth (now part of Pfizer Pharmaceuticals).

Pass it on: Women who've had a hysterectomy can take estrogen-only hormone replacement therapy for about five years without an increased risk in breast cancer, according to a new study.

Rachael Rettner

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.